The underlying object of the present invention has already been discussed in EP-A-0,101,988. According to this document, the outlet pipe and the cleaning piston are heated after each filling of a mold (injection operation), so that the plastics residue which remains adhered to the outlet pipe and to the cleaning piston after filling of the mold is volatilized. However, due to the inertia of heating, the solution which is proposed in EP-A-0,101,988 cannot be used for a rapid operating cycle, i.e. when there is a rapid sequence of injection operations, or can only be used at high cost. For example, during the substantially automated mold filling operations which are presently employed, plastics molds are filled using cycle times of 10 to 30 seconds. The problem also arises of reliably capturing and disposing of the gases produced during the volatilization of the plastics residues.
It has been proposed according to U.S. Pat. No. 4,340,559 that residues which adhere to the side of the outlet pipe are first pushed by means of a stripping ring into the outlet plane of the outlet pipe and that such residues are subsequently removed from the outlet pipe and the cleaning piston by means of a cleaning belt which travels over rollers. The removal of residues as discussed by U.S. Pat. No. 4,340,559 results in problems if substantial hardening of the reactive polyurethane mixture has already occurred before the residue is removed.